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Fair Value Measurements - Fair Values of Financial Assets and Liabilities, Including Derivative Financial Instruments (Details) - USD ($) $ in Thousands | Sep. 30, 2024 | Dec. 31, 2023
--- | --- | ---
Earning assets: |  |
Loans held for sale | $ 0 | $ 0
Trading investments | 54,840 | 54,481
Accrued interest receivable, difference | 129,945 | 68,862
Derivative instruments | 516 | 0
Total earning assets, difference | 3,482,213 | 2,534,325
Interest-bearing liabilities: |  |
Long-term borrowings, difference | 146,758 | 353,822
Derivative instruments | 0 | 370
Total interest-bearing liabilities, difference | 86,844 | 489,912
Excess of net asset fair value over carrying value | 3,569,057 | 3,024,237
Money-market and savings accounts |  |
Interest-bearing liabilities: |  |
Deposits, difference | 6,903 | 68,409
Certificates of deposit |  |
Interest-bearing liabilities: |  |
Deposits, difference | (66,817) | 67,681
Fair Value |  |
Earning assets: |  |
Loans held for sale | 485,701 | 0
Cash and cash equivalents | 4,489,539 | 4,149,838
Trading investments | 54,840 | 54,481
Available-for-sale investments | 2,022,605 | 2,411,622
Accrued interest receivable | 1,667,539 | 1,448,766
Derivative instruments | 600 | 0
Total earning assets | 32,533,025 | 30,836,527
Interest-bearing liabilities: |  |
Long-term borrowings | 5,889,769 | 4,873,690
Accrued interest payable | 98,945 | 105,066
Derivative instruments | 84 | 370
Total interest-bearing liabilities | 27,493,422 | 26,494,693
Fair Value | Money-market and savings accounts |  |
Interest-bearing liabilities: |  |
Deposits | 10,477,366 | 11,134,883
Fair Value | Certificates of deposit |  |
Interest-bearing liabilities: |  |
Deposits | 11,027,258 | 10,380,684
Carrying Value |  |
Earning assets: |  |
Loans held for sale | 485,701 | 0
Cash and cash equivalents | 4,489,539 | 4,149,838
Trading investments | 54,840 | 54,481
Available-for-sale investments | 2,022,605 | 2,411,622
Accrued interest receivable | 1,537,594 | 1,379,904
Derivative instruments | 600 | 0
Total earning assets | 29,050,812 | 28,302,202
Interest-bearing liabilities: |  |
Long-term borrowings | 6,036,527 | 5,227,512
Accrued interest payable | 98,945 | 105,066
Derivative instruments | 84 | 370
Total interest-bearing liabilities | 27,580,266 | 26,984,605
Carrying Value | Money-market and savings accounts |  |
Interest-bearing liabilities: |  |
Deposits | 10,484,269 | 11,203,292
Carrying Value | Certificates of deposit |  |
Interest-bearing liabilities: |  |
Deposits | 10,960,441 | 10,448,365
Private Education Loans |  |
Earning assets: |  |
Loans held for investment, net, difference | 3,352,268 | 2,456,752
Private Education Loans | Fair Value |  |
Earning assets: |  |
Loans held for investment, net | 23,812,201 | 22,229,045
Private Education Loans | Carrying Value |  |
Earning assets: |  |
Loans held for investment, net | 20,459,933 | 19,772,293
FFELP Loans |  |
Earning assets: |  |
Loans held for investment, net, difference | 0 | 8,711
FFELP Loans | Fair Value |  |
Earning assets: |  |
Loans held for investment, net | 0 | 542,775
FFELP Loans | Carrying Value |  |
Earning assets: |  |
Loans held for investment, net | $ 0 | $ 534,064

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